Substrate attach adhesive film, application method and devices incorporating the same

ABSTRACT

A heat activated adhesive film is disclosed, 2-15 mil thick, for bonding a circuit-printed substrate to its support. The adhesive&#39;s main constituent is a thermoplastic organic high polymer containing the elements of carbon and hydrogen, and one or more of the elements oxygen, nitrogen, sulfur, halogen in its repeating unit. The polymer&#39;s Vicat softening temperature is 70°-280° C. Optionally, the adhesive may also contain up to 45 volume percent of a thermally conductive inorganic filler of 0.5-20 micrometer particle size. The attachment process simply consists of heating and pressing the film, sandwiched between the adherent surfaces, above the melting temperature and cooling to ambient.

BACKGROUND OF THE INVENTION

Electronic devices, such as transistors, diodes, resistors, capacitors,transducers, monolithic packages, hybrid microcircuits, contain one orseveral dies or chips bonded onto a rigid substrate with an adhesive.

The die is a semiconductor on which all the active and passivemicrocircuit elements have been fabricated using one or all of thetechniques of diffusion, passivation, masking, photoresist and epitaxialgrowth.

The substrate is the material upon which circuits are printed orfabricated. It is used as a mechanical support but may serve usefulthermal and electrical functions. It may be made of a number of ceramicmaterials, such as alumina, barium titanate, beryllia, boron nitride,porcelain, multilayer ceramic, aluminum nitride, silicon carbide, andother materials such as steatite, glass, sapphire, metal (such as steel)enameled with glass, quartz, epoxy, polyimide, etc.

The substrate, in turn, is bonded to a larger, rigid plate, boardpackage or case, herein referred to as support. It is usually made ofmetal, ceramic, glass or high temperature plastic, providing protection,mechanical support or hermeticity.

Substrates are usually fixed to the support with an epoxy adhesive. Itis currently available as a B-stage epoxy sheet coated on both sides ofa glass fabric for support. The epoxy adhesive presents a great numberof disadvantages:

Since the epoxy is of a tacky, semi-solid consistency, it must have aglass fabric carrier in the middle and a release-coated plastic film onboth sides of the sheet;

It requires freezing or refrigeration for storage;

It has to be cured or polymerized for 1-24 hours at 90°- 200° C.;

It usually contains large quantities of ionic impurities, often up to400 ppm Cl⁻ and Na⁺, which are conducive to corrosivity and hencedecreased reliability of the circuit;

Once the substrate is bonded with the epoxy thermosetting adhesive, itis very difficult to remove it if rework is required for some reason.

The object of this invention is to overcome these difficulties.

SUMMARY OF THE INVENTION

Accordingly, the object of this invention is to provide a substrateattach self-supporting adhesive film requiring no glass fabric, norelease liner, no refrigeration for storage and no long curing step. Another object is to reduce ionic impurities, and improve reworkability.

A further object is a simple application method and an electronic deviceincorporating the adhesive of this invention.

These and other advantages are realized in the present invention bymeans of a new adhesive film, 2-15 mil thick, comprising 55-100 percent,by volume, of a thermoplastic organic high polymer containing eitheroxygen, nitrogen, sulfur or halogen in addition to carbon and hydrogenand having Vicat softening temperature of 70°-280° C., and an inertinorganic particulate matter with thermal conductivity exceeding 10W/m-° C. and average particle size of 0.5-20 micrometer, preferably 1-12micrometer.

Substrate attachment comprises the following steps:

The adhesive film is sandwiched between the substrate and its support;

The film is heated to its melt and the substrate is pressed against themolten adhesive for 0.01-10 minutes;

The pressure is released and the bonded assembly is cooled to ambienttemperature.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, effective compositions,processes and devices can be produced to bond substrates with printedcircuits to their support, and overcoming the difficulties encounteredwith the state of the art epoxy adhesive films. Some of the improvementsprovided by this invention are:

The adhesive is a self-supporting, non-tacky solid film needing no glassfabric support and release liner;

It has long shelf life at room temperature requiring no refrigeration;

Only short processing times are necessary since there is no longreaction type curing;

The adhesive of the present invention has generally considerably betterionic purity then epoxies;

Because of the adhesive's thermoplastic nature reworking is easy. Onehas only to apply heat and remove the die with a pair of tweezers.

The new adhesive of this invention for bonding a circuit printedsubstrate to its support is a self supporting, heat activated film of2-15 mil in thickness (1 mil =0.001 inch). If the thickness is less than2 mil, the resulting bond may have voids caused by unevenness and camberof the bonded surfaces, especially with larger substrates. Voidsdecrease adhesive bond strength and heat transfer. On the other hand,adhesive thickness in excess of 15 mil takes up too much space in amicrocircuit device and insulates it thermally. The preferred range ofthickness is 3-12 mils.

The composition of the adhesive comprises 55-100 percent, by volume, ofa thermoplastic organic high polymer containing the element of carbonand hydrogen, and one or more of the elements oxygen, nitrogen, sulfur,halogen in its repeating unit, and having Vicat softening temperature of7°-280° C., and 0-45 percent, by volume, of an inorganic particulatematter with thermal conductivity in excess of 10 W/m-° C. and averageparticle size of 0.5-20 micrometer.

The term high polymer is recognized to designate a high molecular weightpolymer, one whose basic properties don't change significantly byfurther incremental increases in molecular weight.

In order to form a good adhesive bond, the polymer must contain in itsrepeating units, in addition to the elements of carbon and hydrogen, oneor more of the polar elements of oxygen, nitrogen, sulfur and halogen.Such elements may be part of the main polymeric chain or attached to itas a side chain in polar groups by covalent bonds. Examples of polargroups in the polymer are: carboxyl, ester, ether, nitrile, imide,amide, sulfone, chloride, fluoride, bromide.

In order to be heat activated, the polymer must be thermoplastic sinceactivation takes place above the softening temperature. This is measuredby the ASTM method D1525 (IS0-R306) - 1 kg load, and called the Vicatsoftening temperature. The higher this temperature, the highertemperature the adhesive stays functional in the electronic device. Formilitary, space and certain commercial applications, Vicat softeningtemperatures higher than 150° C. is most desirable. Exceedingly hightemperatures, on the other hand, render the substrate attachment processdifficult. The polymers of this invention are operative at 70°-280° C.Vicat softening temperature range. The polymers listed below andabbreviated as PSO, PCT, PBT, PAR, PEI, and PVDF melt at 150°-280° C.and PUR, PAC, APES, CELL and ECOP melt at 70°-150° C.

Examples of thermoplastic organic high polymers according to thisinvention are:

Sulfone polymers, (PSO), such as polysulfone, polyethersulfone,polyarylsulfone, and modified polysulfone. Many of these polymers may berepresented by the formulas of their repeating units --O--C₆ H₄ --SO₂--C₆ H₄ --and C₆ H₄ --SO₂ --C₆ H₄ --O--C₆ H₄ C(CH₃)₂ --C₆ H₄ --

Polycarbonates, (PCT), such as --COO --C₆ H₄ --C(CH₃)₂ --C₆ H₄ --O--

Polyesters, such as polybutylenete-repthalate (PBT), aliphaticpolyesters (APES);

Polyetherimides, (PEI), such as ##STR1##

Polyarylates, (PAR), such as the bisphenol A ester of terephthalic acid;

Polyvinylidene fluoride (PRDF);

Copolymers of ethylene (ECOP) with minor amounts (less than about 1/3)of maleic anhydride, methyl or ethyl acrylate, 10-35 weight percentvinyl acetate, or 3-10 weight percent of a carboxylic acid containingmonomer optionally neutralized with a metal.

Polyamides or nylons;

Cellulose esters such as cellulose acetate, propionate, nitrate, andethers, such as ethyl cellulose (CELL);

Acrylic polymers, (PAC) which are homo or block copolymers of alkylesters of acrylic and methacrylic acid, optionally with up to 10% ofacrylic and methacrylic acid and up to 30% styrene or vinyl acetate,

Polyurethanes, (PUR), prepared from diisocyanates and glycols, such aspolyesterglycols or polyetherglycols,

The adhesive film may be made entirely of the polymer of this inventionor compounded with a thermally conductive particulate matter, calledfiller. The amount of filler compounded with the polymer depends on anumber of factors, such as the desired conductivity, particle size andother properties of the compounded adhesive. For high conductivityapplications filler amounts higher than about 45 percent, by volume,results in deterioration of adhesive properties. Therefore, theinvention is operative at polymer/filler volume percents of 5-100/0-45,preferably 40-100/0-40. For high conductivity applications 25-45 volumepercent filler is preferred.

Commercial films operative in this invention at zero percent fillercontent include PUR, PAC, ECOP, CELL and the high melting films PSO,PCT, PEI, PBT, PVDF.

The filler of this invention is a particulate material with thermalconductivity higher than about 10 W/m-° C. Such material is, for examplethe metals silver, gold, the ceramics alumina, beryllia, silica, siliconcarbide, barium titanate, steatite, boron nitride, aluminum nitride andsteatite. It must have an average particle size of 0.5-20 micrometer,preferably 1-12 micrometers.

The filler's high thermal conductivity, particle size and proportionprovides high thermal conductivity for the adhesive, generally in therange of 0.7-4 W/m-° C. Without the filler the thermal conductivity isin the range of 0.2-0.3 W/m-° C.

Having thus defined the composition of the invention the followingparagraphs illustrate preparation. A bonding technique is alsodescribed.

The adhesive film is prepared either by a solution process or a meltprocess.

In the solution process the adhesive is prepared by first dissolving thepolymer in a solvent. This may be conveniently accomplished by slowlyadding the polymer, in a powder, chip or pellet form to a stirringsolvent. High speed stirring, low particle size and higher temperaturesspeed up the rate of dissolution. It may take 0.5 to 24 hours to obtaina clear solution.

Continuing stirring the polymer solution, other ingredients may then beadded:

Antifoam, 0-0.5 weight percent, based on solvent;

Wetting agent, 0-2 weight percent, based on filler;

Filler of this invention, 0-45 volume percent of the adhesive;

Antioxidant, 0-2 weight percent of the polymer.

Stirring may be slow or fast depending on viscosity and stirring bladegeometry. High speed stirring for 15-60 minutes may be necessary tothoroughly disperse the fillers. If excessive amount of air is entrappedin the adhesive, it is deairated by gentle tumbling or pulling vacuumover it. It is then cast on a flat surface, preferably coated with arelease compound, and the solvent is driven off in an oven.

Commercial polymers that can be easily compounded in solution withfillers of this invention include PSO, PAC, PUR, PEI, APES, and CELL.

In the melt process the polymer is melted and optionally mixed with aconductive filler and other ingredients at 80°-380° C. It is thenextruded at these temperatures by forcing it through a fine slot anddrawing to form a film.

The adhesive bonding process comprises the following simple steps:

Positioning the adhesive film between the substrate and its support.More than one layer of film may be used;

Heating the film to its melt and pressing the substrate against themolten adhesive. Heat may be applied with a hot plate, oven, orinduction heating. The temperature of the molten film depends on thepolymer, but should be such as to obtain good flow to wet the adheringsurfaces.

Pressure to the substrate may be applied by hand or a machine withscrubbing action. The pressure ought to be great enough and the time forpressing long enough for the adhesive to contact and wet the bondingsurfaces without squeezing the adhesive out of the bond line. The higherthe pressure, the lower the temperature may be for bonding, for lesstime. Useful pressure and time parameters are 0.2-20 pounds per squareinch and 2 seconds to 20 minutes.

The pressure is then released and the bonded assembly is cooled toambient temperature to form a strong bond between the substrate and itsupport.

EXAMPLE 1

A 5 mil thick polycarbonate film, sold under the trade name Lexan 8010by General Electric Co., was analyzed and found containing less than 1ppm Cl⁻ and Na⁺. It had a Vicat softening temperature of 155° C.

A 1"×1"piece of the film was cut and positioned between a 30 mil thickalumina substrate and a 40 mil thick gold plated Kovar (commercial alloyof Fe+Co+Ni) hybrid case of similar size. The sandwiched structure wasplaced on a 300° C. hot plate and pressed down with a preheated halfpound weight. In 3 minutes the weight was lifted and the structure takenoff the hot plate to cool.

The properties of adhesive and the bond formed were measured perMIL-STD-883C, Method 5011.1 and the following excellent results wereobtained:

    ______________________________________                                        Die Shear Strength                                                            At 25° C.     25 meg. N/m.sup.2                                        At 150° C.    12 meg. N/m.sup.2                                        At 25° C. following Sequential                                                              37 meg. N/m.sup.2                                        Environment Testing                                                           (Including 1000 hours at                                                      150° C. in air oven and                                                temperature cycling 100                                                       times between - 65 and                                                        150° C.)                                                               Thermal Conductivity 0.20 W/m-°C.                                      Coef. Thermal Expansion                                                                            60 ppm/°C.                                        Volume Resistivity   15.sup.16 Ohm-cm                                         Dielectric Constant  3.17                                                     Dissipation Factor   0.001                                                    Weight Loss (TGA) at 300° C.                                                                0.2%                                                     Constant Acceleration Test                                                                         Pass 15000 g                                             Reworkability        Just heat to 180° C. and                                               remove substrate with                                                         tweezers                                                 Shelf Life at Room Temperature                                                                     12 months plus                                           ______________________________________                                    

EXAMPLES 2-3

Two commercial films, a 4 mil thick copolymer of ethylene with 6.5weight percent acrylic acid, (EAA), having Vicat softening temperatureof 85° C., and a 12 mil thick film of polybutylene terephtholate (PBT)with Vicat softening temperature of 220° C. were analyzed and foundcontaining less than 1 ppm Cl³¹ and Na³⁰. The films were cut toappropriate size to bond alumina substrates to gold plated Kovar caseswith EAA, and to nickel plated Kovar cases, respectively, with PBT.Substrate sizes were 1"×1/2" for EAA and 1.5"×2" for PBT. The bondingand testing methods were the same as in Example 1 and the followingresults were obtained.

    ______________________________________                                                         EAA      PBT                                                 ______________________________________                                        Hot plate temperature, °C.                                                                105        290                                             Force applied on bonding, psi                                                                    9          0.3                                             Bonding time at indicated                                                                        0.2        6                                               temp., minutes                                                                Shear Adhesion, meg-N/m.sup.2                                                 at 25° C.   12         31                                              at 150° C.  --         12                                              at 25° C. following                                                                       --         38                                              Sequential                                                                    Environment Testing                                                           Dielectric Constant                                                                              2.36       3.2                                             Volume Resistivity, Ohm-cm                                                                       10.sup.17  10.sup.16                                       Dissipation Factor (10.sup.7 Hz)                                                                 0.0014     0.02                                            Thermal Conductivity at                                                       121° C., W/m-°C.                                                                   0.19       0.17                                            Weight Loss (TGA) at 300° C., %                                                           --         0.5                                             Coefficient of Thermal                                                                           --         89                                              Expansion, -55 to 150° C.,                                             ppm/°C.                                                                Reworkability      Excellent  Excellent                                                          at 105° C.                                                                        at 225° C.                               Constant Acceleration Test                                                                       Pass 15000 g                                                                             Pass 15000 g                                    Shelf life of adhesive at                                                                        12+        12+                                             room temperature, months                                                      ______________________________________                                    

EXAMPLE 4

100 g polysulfone pellets, having a Vicat softening temperature of 190°C. and sold under the trade designation Udell 3703 by Amoco ChemicalCo., was stirred into 250 g. o-dichlorobenzene heated in a 500 ml. glassbeaker to 80° C. with a hot plate to obtain a clear, viscous solution inabout 4 hours. The total solids was 28.6% and the viscosity 20,000 cps(Brookfield viscometer, 3.75 per second shear rate). The followingingredients were then stirred into the solution, in order:

    ______________________________________                                        o-dichlorobenzene,     50.0                                                   Dehydran ARA 7219, an antifoam                                                                       0.05                                                   sold by Henkel Corp.                                                          P-211 silver flake-powder mixture,                                                                   376.0                                                  average particle size 5 micrometer,                                           thermal conductivity = 419 W/m-°C.,                                    sold by Englehard Corp.                                                       ______________________________________                                    

The mixture was further stirred at 2,000 rpm for half an hour. The totalsolids of the paste was 61 weight percent and viscosity 12,600 cps at37.5 per second shear rate. The silver constituted 79 weight percent and31 volume percent of the adhesive's solids.

The adhesive was then drawn down between two rolls on a silicone coatedpaper release liner and dried in a vacuum oven at room temperature for 2hours and at 150° C. for 16 hours to a dry thickness of 3 mil.

The properties of the adhesive and the bond were measured as in Example1 to obtain the following results:

    ______________________________________                                        Shear Adhesion                                                                at 25° C.     26 meg-N/m.sup.2                                         at 150° C.    19 meg-N/m.sup.2                                         at 25° C. following Sequential                                                              38 meg-N/m.sup.2                                         Environment Testing                                                           Electrical Conductivity                                                                            10.sup.-4 Ohm-cm.                                        at 25° C.                                                              Thermal Conductivity at 121° C.                                                             3 W/m-°C.                                         Weight Loss (TGA) at 300° C.                                                                0.3%                                                     Coefficient of Thermal                                                                             40 ppm/°C.                                        Expansion, -55 to 150° C.                                              Reworkability        Excellent - just heat                                                         to 220° C. and remove                                                  substrate with tweezers                                  Shelf life of adhesive at                                                                          12 months plus                                           room temperature                                                              Leachable Chloride   5 ppm                                                    Leachable Sodium     1 ppm                                                    ______________________________________                                    

EXAMPLE 5

350 g. of polysulfone solution prepared as in Example 4 was compoundedas in Example 4 with alumina as follows to provide thermal conductivityand maintaining electrical insulating properties:

    ______________________________________                                        Dehydran ARA 7219    0.05                                                     Alumina, particle size                                                                             200                                                      8 micrometer, thermal                                                         conductivity = 34 W/m-°C.                                              Irganox 1010 antioxidant                                                                           0.2                                                      ______________________________________                                    

The total solids of the paste was 55.2 weight percent, and theBrookfield viscosity 42,000 cps at 37.5 per second shear rate. Thealumina constituted 67 weight percent and 38 volume percent of theadhesive's solids.

The adhesive was applied and tested as in Example 4, and the followingexcellent properties were obtained:

    ______________________________________                                        Die Shear Strength                                                            At 25° C.   28 meg. N/m.sup.2                                          At 150° C.  21 meg. N/m.sup.2                                          At 25° C. following Sequential                                                            44 meg. N/m.sup.2                                          Environment Testing                                                           Thermal Conductivity                                                                             0.96 W/m-°C.                                        Coef. Thermal Expansion                                                                          40 ppm/°C.                                          Volume Resistivity 10.sup.14 Ohm-cm                                           Dielectric Constant                                                                              3.9                                                        Dissipation Factor 0.01                                                       Weight Loss (TGA) at 300° C.                                                              0.3%                                                       Reworkability      Heat to 220° C. and remove                                             substrate with tweezers                                    Shelf Life at Room Temperature                                                                   12 months plus                                             ______________________________________                                    

EXAMPLES 6-10

Adhesives of various compositions were prepared comprising theingredients listed in Table I following the procedure described inExample 4. They were then used to bond circuit printed substrates totheir support and tested as in Example I. All the adhesives hadexcellent properties, e.g., self supporting, not tacky at roomtemperature, long shelf life, high ionic purity, excellent adhesion goodthermal conductivity, and excellent reworkability.

                  TABLE 1                                                         ______________________________________                                        ADHESIVE COMPOSITION AND PROPERTIES OF                                        EXAMPLES 6-10                                                                 Polymer       Conductive Filler                                                                            Sub-    Sup-                                     Ex.  Type   Vicat   %   Type Size TC   %   strate                                                                              port                         ______________________________________                                        6    PUR     85     39  Au   5    298  61  BeO   SS                           7    PEI    220     35  AlN  9    180  65  AlN   Kovar                        8    PES    180     41  SiO.sub.2                                                                          15   16   59  Al.sub.2 O.sub.3                                                                    PEI                          9    PAR    150     29  BN   0.8  50   71  BN    Kovar                        10   PAC     55     32  SiC  2    70   68  SiC   Kovar                        ______________________________________                                         Abbreviations                                                                 Size: Particle Size in Micrometers;                                           %: Volume percent in adhesive film;                                           TC: Thermal Conductivity W/m°C.;                                       Vicat: Vicat Softening Temperature, °C.;                               PUR: Polyurethane;                                                            PEI: Polyetherimide;                                                          PES: Polyethersulfone;                                                        PAR: Polyarylate;                                                             PAC: Methylmethacrylate/Ethylacrylate/Acrylic Acid Copolymer, T.sub.g =       70° C.;                                                                SS: Stainless steel;                                                          Kovar: Commercial Alloy of Fe + Co + Ni.                                 

I claim:
 1. A heat activated adhesive film, 2-15 mil thick, for bondinga circuit-printed substrate to its support, said film comprising:55-100percent by volume of a thermoplastic organic high polymer containing theelements carbon and hydrogen, and one or more of the elements oxygen,nitrogen, sulfur, halogen in its repeating unit, and having Vicatsoftening temperature of 70°-280° C., and 0-45 percent, by volume, of aninorganic particulate matter with thermal conductivity in excess of 10W/m-° C. and average particle size of 0.5-20 micrometer.
 2. A film as inclaim 1 wherein said polymer has a Vicat softening temperature of150°-280° C.
 3. A film as in claim 1 wherein said polymer has a Vicatsoftening temperature of 70°-150° C.
 4. A film as in claim 1 wherein theamount of said filler is zero percent.
 5. A film as in claim 1 whereinthe amount of said filler is 25-45 volume percent.
 6. A film as in claim2 wherein said polymer is selected from the group consisting of sulfonepolymers, polycarbonates, polyetherimides, polyvinylidene fluoride,polyarylates and polybutylene terephthalate.
 7. A film as in claim 3wherein said polymer is selected from the group consisting of aliphaticpolyesters, cellulose ethers or esters, polyacrylates, polyurethanes andethylene copolymers with minor amounts of acrylic or methacrylic acid,maleic anhydride, methyl or ethyl acrylate or vinyl acetate.
 8. A filmas in claim 4 wherein said polymer is selected from the group consistingof polycarbonates, sulfone polymers, polyetherimides, polyvinylidenefluoride, polybutylene terephthalate.
 9. A film as in claim 4 whereinsaid polymer is selected from the group consisting of polyurethane,polyacrylates, cellulose esters or ethers, ethylene copolymers withminor amounts of acrylic or methacrylic acid, methyl or ethyl acrylate,maleic anhydride or vinyl acetate.
 10. A film as in claim 5 wherein saidpolymer is selected from the group consisting of sulfone polymers,aliphatic polyesters, polyetherimides, esters and ethers of cellulose,polyacrylates and polyurethanes.
 11. A film as in claim 5 wherein saidfiller is a ceramic selected from the group consisting of alumina,aluminum nitride, boron nitride, silicon carbide, silica and bariumtitanate.
 12. A film as in claim 5 wherein said filler is silver orgold.
 13. A film as in claim 10 wherein said polymer is a sulfonepolymer and said filler is silver or gold.
 14. A film as in claim 10wherein said polymer is a polyacrylate and said filler is silver.
 15. Afilm as in claim 10 wherein said polymer is a polyurethane and saidfiller is silver.
 16. A film as in claim 10 where said polymer is analiphatic polyester and said filler is silver.
 17. A film as in claim 10wherein said polymer is a polyetherimide and said filler is silver orgold.
 18. A film as in claim 10 wherein said polymer is a celluloseester or ether and said filler is silver.
 19. A film as in claim 10wherein said filler is selected from the group consisting of alumina,aluminum nitride, boron nitride, silicon carbide, barium titanate.
 20. Amethod of bonding a circuit-printed substrate to its support comprisingthe steps:Positioning a heat activated adhesive film between saidsubstrate and support; Heating the film to its melt and pressing thesubstrate against the molten adhesive; Releasing the pressure andcooling to ambient temperature;Said adhesive film being 2-15 mil thickand comprising: 55-100 percent, by volume, of a thermoplastic organichigh polymer containing the elements carbon and hydrogen, and one ormore of the elements oxygen, nitrogen, sulfur, halogen in its repeatingunit, and having Vicat softening temperature of 70°-280° C., and 0-45percent, by volume, of an inorganic particulate matter with thermalconductivity in excess of 10 W/m°C. and average particle size of 0.5-20micrometer.
 21. A microelectronic device containing a circuit printedsubstrate bonded to a support with a 2-15 mil thick heat activatedadhesive film comprising:55-100 percent by volume of a thermoplasticorganic high polymer containing the elements carbon and hydrogen, andone or more of the elements oxygen, nitrogen, sulfur, halogen in itsrepeating unit, and having vicat softening temperature of 70°-280° C.,and 0-45 percent, by volume, of an inorganic particulate matter withthermal conductivity in excess of 10 W/m-°C. and average particle sizeof 0.5-20 micrometer.
 22. A device as in claim 21 wherein said substrateis a ceramic selected from the group consisting of alumina, beryllia,porcelain, aluminum nitride, boron nitride, silicon carbide and saidsupport is a metal or ceramic.